ESTABLISHING RELATIONSHIPS BETWEEN NUTRIENTS AND ALGAE “IMPAIRMENT” IN STREAMS: THE CHALLENGE OF NUTRIENT TMDLS

There is an inherently high degree of variability and complexity to nutrient-algae relationships and a number of complicating environmental variables that affect our ability to implement nutrient Total Maximum Daily Loads (TMDLs), in general, and in streams, in particular. The ambient biomass of attached algae or macrophytes in a stream reflects current and past exposure to current scour, sunlight, and micro-habitat conditions, all of which may affect standing crop as much or more than ambient concentrations of nitrogen or phosphorus. The approach of establishing TMDL nutrient trigger values that are protective of levels of impairment due to algal density is often too simplistic. Key issues are 1) What actually constitutes impairment?, 2) How do we establish functional relationships between impairment, algae densities, and nutrients?, 3) How can we incorporate seasonal and spatial heterogeneity into the nutrient TMDL process, and 4) Can reference condition comparisons be useful? Site-specific, calibrated models are needed to effectively relate nutrients to impairment.Case studies of nutrient TMDLs from across the United States demonstrate the wide variety of approaches used to link nutrients to impairment. However, rarely are quantitative relationships experimentally determined between water quality criteria, appropriate measures of impairment, and nutrient concentrations. Instead, arbitrary reductions in load or literature-based, generic modeled relationships are often used to set TMDL trigger values.We present a case study of the analysis of a 20 year record of attached algae (Cladophora glomerata) and nutrients in Malibu Creek, in an attempt to understand the relationships between impairment due to algae coverage and nutrient dynamics. The creek is listed as impaired due to excessive algae growth. However, results of the analysis of the long-term data set indicate a lack of positive relationships between algae coverage and in-stream nutrient concentrations. Nutrient concentrations do not appear to be limiting to algae growth in the creek. Nevertheless, periphyton algae vary positively with light and negatively with winter season current scour. The algae occur primarily in the low-flow, sunlit summer months and primarily in open, unshaded areas of the creek. Physical factors appear to be more important than variations in nutrient chemistry in determining the variability of algae growth in the creek. This is a typical pattern for Cladophora in other systems. The case study of Malibu Creek demonstrates the importance of site specific studies and the difficulties in assigning nutrient levels to control algal growth in streams.